[MCA] Speedup ResourceManager queries. NFCI

When a resource unit R is released, the ResourceManager notifies groups that
contain R. Before this patch, the logic in method ResourceManager::release()
implemented a potentially slow iterative search of dependent groups on the
entire set of processor resources.
This patch replaces that logic with a simpler (and often faster) lookup on array
`Resource2Groups`.  This patch gives an average speedup of ~3-4% (observed on a
release build when testing for target btver2).
No functional change intended.

llvm-svn: 353301